| As for the severity of the surface water pollution, water ecological balance was break and aquatic community had altered, tolerance species such as Chironomid larvae and Limnodrilus increased a lot in the water. While the surface water mentioned above are used as sources for the supply of drinking-water, it is possible for the small animals passing through water treatment works and finding way to the storage reservoirs, if the barriers are not completely effective.The dominant organisms in the drinking-water supply system of Xi'an district was studied. It was found that a generation of Chironomid artificial cultured in the lab last 30 days at least, the larva stage was where most of the chironomid life cycle was spent, about 25 days, the period for Daphnia was 6 days. Tubifex growth in preference condition needed 55-90 days. Chironomid larvae could survive some days in the water of poor nourishment, strongly acidity, strong basicity or low temperature. Daphnia could adapt to a wide range of temperature and dissolved oxygen of the water, whereas need proper pH value. Tubifex were subjected to low dissolved oxygen, but it couldn't survive in the weak acidity and weak basicity solution. The inactivation efficiency on Chironomid larvae of five oxidizing agents was studied. The results showed that in the experimental condition liquid chlorine, hydrogen peroxide and potassium permanganate had little effect on the larvae, with 1 and 10 mg·L-1 dosage. Extension of contact time and change of pH value couldn't enhance the death rate much more.The larvae had complicate organization and better resisting power than microorganism, so the three chemical reagents can't kill the larvae completely and effectively. Chlorine dioxide and ozone possessed more favorable inactivation effect on Chironomus larvae compared with other oxidants. Organic substance in the water consumed some chlorine dioxide and made the mortality rate decrease. PH value had little influence on the inactivation effect of chlorine dioxide, but water temperature had a lot on it. The death rate increased wih the water temperature increasing. The optimum CT (product of the dosage C and the contact time 7) of chlorine dioxide in the experiment was 400 to 600mg·min·L-1, the limited value ofC was 4 to 10mg·L-1 and T was 30 to 300min. Ozone may kill the whole larvae in low dosage and short contact time. The Chironomus larvae wouldn't die entirety within 24h with the ozone dose of 0.5 and 0.8mg·L-1. First order reaction was used to describe the process of ozone killing the larvae, among which existed hysteresis phenomenon.A comparision was also made on the inactivation efficiency of Daphnia and Tubifex killed by five oxidants. The sequence of the chemical agents' killing rate on Daphnia from high to low respectively was ozone, chlorine dioxide, liquid chlorine, potassium permanganate and hydrogen peroxide, on the Tubifex was ozone, liquid chlorine, chlorine dioxide, potassium permanganate and hydrogen peroxide. The extension contact time contributed to the exaltation of oxidants' effect to the animals. The inactivation rate changed wih the water pH value changing. The inactivation mechanism was different from each reagent. The chemical agent with greater oxidability would kill Tubifex more quickly than others.
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